Storage stable, one-package heat curable polyurethane adhesive compositions

ABSTRACT

New and improved storage stable, one package, heat curable polyurethane adhesive compositions are disclosed which comprise: 
     (a) a mixture of or a prepolymer adduct of: 
     (i) an excess of a poly aliphatic isocyanate compound of the formula: ##STR1##  wherein R and R&#39; are each, independently, hydrogen, alkyl or substituted alkyl and X is a bridging means selected from divalent aliphatic, cycloaliphatic or aromatic groups; and 
     (ii) at least one monomeric or polymeric compound containing a plurality of hydroxyl groups; in admixture with 
     (b) a stoichiometric amount of a curing agent comprising a monomeric or polymeric compound having at least three hydroxyl groups at least one of which is a secondary hydroxyl group; and 
     (c) optionally, an inert filler. 
     The compositions are curable upon exposure to elevated temperatures of from about 100°-200° C. to form high tensile strength, high adhesive strength bonds to a variety of substrates. The compositions have extended pot lives of up to six weeks and exhibit storage stability for up to six months.

BACKGROUND OF THE INVENTION

The present invention relates to new and improved polyurethane adhesivecompositions. More particularly, it relates to one package polyurethaneadhesives based upon poly aliphatic isocyanate compounds which arestorage stable for prolonged periods of time and which are curable byheat to form high bond strength adhesive joints which exhibit excellentadhesion to a variety of substrates.

In this application, the term "aromatic isocyanate" refers to an organicisocyanate compound wherein the isocyanate group(s) is bonded directlyto a carbon atom of an aromatic nucleus. By "aliphatic isocyanate" ismeant an organic isocyanate compound wherein the isocyanate group orgroups are bonded directly to an aliphatic carbon atom. By"polyaliphatic isocyanate" is meant an aliphatic isocyanate having morethan one such isocyanate bonded to aliphatic carbon atoms in themolecule.

Polyurethane adhesives are widely used for bonding various metallic,plastic and glass substrates because they form good adhesive bonds withthese substrates. Generally, prior art polyurethane adhesivecompositions comprise two component systems in which adiisocyanate-containing component and a polyol containing component areproperly measured and mixed immediately prior to applying the adhesiveto the substrate. These compositions have extremely rapid gellationtimes upon standing at room temperature of a few minutes to a few hours.After gellation, the compositions cannot be further used as adhesivesbecause they are difficult, if not impossible to apply and do not bondsatisfactorily to substrates.

In U.S. Pat. No. 4,119,594, thermosetting polyurethane compositionshaving improved pot lives useful for forming injection molded articlesare disclosed. These compositions comprise diphenylmethane diisocyanateor a liquid isocyanate-terminated reaction product of diphenylmethanediisocyanate and a diol; about 40 to 85 equivalents of hydroquinonedi-(beta-hydroxyethyl) ether, a liquid polyol having hydroxyfunctionality of greater than two and an equivalent weight of about2,000 and a catalytic amount of zinc stearate, said composition havingan excess of isocyanate groups over hydroxyl groups of up to 10%. Thecompositions are described as stable at room temperature for up toseveral hours and cure upon injection molding at temperatures of fromabout 120° to 170° C. to form satisfactory molded articles.

It has now been discovered that one package heat curable polyurethaneadhesive compositions exhibiting good long-term storage stability in aclosed container and useful pot lives at room temperature of up to aboutsix months are provided by a mixture or a prepolymer solution ofsecondary or tertiary polyaliphatic isocyanate compounds and astoichiometric amount of a diol, polyol, or a mixture of diols andpolyols. The adhesive compositions upon heat curing form strong adhesivebonds or joints for a variety of substrates and the inconveniencesassociated with the measuring, mixing and use of the prior arttwo-component systems are avoided.

SUMMARY OF THE INVENTION

In accordance with the present invention, new and improved storagestable, one package, heat curable polyurethane adhesive compositions areprovided in the form of compositions comprising:

(a) a mixture of or a prepolymer adduct of:

(i) a poly aliphatic isocyanate compound having the formula: ##STR2##wherein R and R' are each, independently, selected from hydrogen, alkylor substituted alkyl and X is a bridging means selected from aliphatic,cycloaliphatic or aromatic groups; and

(ii) a monomeric or polymeric compound containing a plurality ofhydroxyl groups, wherein component (ii) is added in an amount so as toprovide less than a stoichiometric hydroxyl equivalent to the aliphaticisocyanate equivalents in (a)(i), preferably in an amount such thatcomponent (a) possesses an overall stoichiometric excess of polyaliphatic isocyanate groups over hydroxyl groups of about 5% to about15% by weight of the total polyaliphatic isocyanate content of (a)(i);in admixture with

(b) a curing agent comprising a monomeric or polymeric compoundcontaining at least three hydroxyl groups, at least one of said hydroxylgroups being a secondary hydroxyl group and preferably not more than twoof said hydroxyl groups being primary hydroxyl groups, said curing agentbeing present in an amount to provide sufficient additional hydroxylgroups to provide a stoichiometrically equivalent amount of hydroxylgroups to the stoichiometric excess of aliphatic isocyanate equivalentspresent in Component (a); and

(c) optionally, an inert filler.

In preferred embodiments, the new and improved adhesive compositions ofthe present invention may comprise:

(a) a mixture of or a prepolymer adduct of

(i) α, α, α, α'-tetramethylxylylene diisocyanate; and

(ii) less than a stoichiometric equivalent amount of ahydroxyl-terminated polyester having a molecular weight of from about100 to about 2000; in admixture with

(b) a curing agent selected from the group consisting of glycerol,1,2,6-hexanetriol, and polyol compounds containing at least onesecondary hydroxyl group; and

(c) from about 10 to about 60 parts by weight, based upon 100 parts byweight of (a) and (b) combined, of a fumed colloidal silica filler.

In accordance with another aspect of the present invention, a new andimproved method for preparing a storage stable, one package,heat-curable polyurethane adhesive composition is provided, said methodcomprising

(a) forming a liquid mixture of a poly aliphatic isocyanate compound ofthe formula: ##STR3## wherein R and R' are each, independently, selectedfrom hydrogen, alkyl and subtituted alkyl and X is a bridging meansselected from divalent aliphatic, cycloaliphatic and aromatic groups, ora prepolymer adduct of said isocyanate possessing polyisocyanatefunctionality in a suitable solvent;

(b) adding to (a), a liquid mixture of at least one monomeric orpolymeric compound containing a plurality of hydroxyl groups, optionallyin a suitable solvent, in an amount so as to provide less than astoichiometrically equivalent amount of hydroxyl groups to theisocyanate groups present in (a) to provide a mixture or prepolymercontaining from about 5% to about 15% by weight of a stoichiometricexcess aliphatic isocyanate equivalents over hydroxyl equivalents in theliquid mixture;

(c) thereafter, adding a curing agent comprising a monomeric orpolymeric compound containing at least three hydroxyl groups, at leastone of which is a secondary hydroxyl group, in an amount sufficient toprovide an approximately stoichiometrically equivalent amount ofhydroxyl groups to said excess of aliphatic isocyanate groups in theliquid mixture of step (b); and

(d) optionally, adding an effective amount of an inert filler to obtaina storage stable one package heat curable polyurethane adhesivecomposition.

In accordance with a further aspect of the present invention, there areprovided new and improved articles of manufacture comprising a pluralityof work pieces having at least a part of a surface portion of one workpiece in close proximity to another workpiece and having an adhesivejoint therebetween being formed of cured polyurethane composition asdefined above, as well as a method for bonding articles to produce sucharticles of manufacture.

Other objects and advantages of the present invention will becomeapparent from the following detailed description of the invention inconjunction with the illustrative working examples.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with these and other objects, the present inventionfirstly comprises as component (a) a mixture of, or a prepolymer adductformed by reaction of:

(i) a poly aliphatic isocyanate compound; and

(ii) at least one monomeric or polymeric compound containing a pluralityof hydroxyl groups.

The aliphatic polyisocyanate compounds for use herein as component(a)(i) may be monomeric or polymeric compounds having a plurality ofaliphatic isocyanate terminal groups. More particularly, component(a)(i) may comprise at least one poly aliphatic isocyanate compound ofthe formula: ##STR4## wherein R and R' are each, independently, selectedfrom hydrogen, alkyl and substituted alkyl and X is a bridging meansselected from divalent aliphatic, cycloaliphatic, and aromatic groups.

Examples of poly aliphatic isocyanate compounds for use in the adhesivecompositions of this invention include:

α, α, α', α'-tetramethyl-1-3-xylylene diisocyanate,

α, α, α', α'-tetramethyl-1,4-xylylene diisocyanate,

methylenebis(4-cyclohexylisocyanate),

isophorone diisocyanate,

1,4-cyclohexylene diisocyanate, and the like.

The especially preferred diisocyanate is α, α, α',α'-tetramethyl-1,3-xylylene diisocyanate, hereinafter referred to asTMXDI.

In addition to the above described monomeric aliphatic isocyanatecompounds, polymeric materials derived in part from these aliphaticisocyanates and possessing terminal aliphatic isocyanate groups may alsobe used as component (a)(i). The polymeric materials containingaliphatic isocyanate terminal groups are polymeric adducts or urethaneprecursors, formed by reacting an excess of poly aliphatic diisocyanatecompound, generally 1 molar equivalent, with a polyol compound,generally 0.3 to about 0.7 molar equivalents, optionally in a suitablesolvent, at temperatures of about 70°-80° C. for several hours, althoughif a catalyst is used, such as an organic tin catalyst, in an amount offrom about 0.01 to 0.1 percent by weight, reaction times may beconsiderably reduced.

Suitable polyols which may be reacted with the polyaliphatic isocyanatecompounds to form isocyanate terminated urethane precursors include:monomeric polyols such as ethylene glycol, propylene glycol, diethyleneglycol, triethylene glycol, dipropylene glycol, trimethylene glycol, and1,3- and 1,4- butanediol.

Polyether polyols, which are obtained by effecting addition of one type,or two types or more, among, for example, ethylene oxide, propyleneoxide, butylene oxide, tetrahydrofuran, and styrene oxide, with forexample, water, ethylene glycol, propylene glycol, diethylene glycol,triethylene glycol, dipropylene glycol, trimethylene glycol, 1,3- and1,4- butanediol, 1,5-pentanediol, 1,2-hexylene glycol, 1,10-decanediol,1,2-cyclohexanediol, 2-butene-1,4-diol, 3-cyclohexane-1,1-dimethanol,4-methyl-3-cyclohexane-1,1-dimethanol, 3-methylene-1,5-pentanediol,(2-hydroxyethoxy)-1-propanol, 4-(2-hydroxyethoxy)-1-butanol,5-(2-hydroxypropoxy)-1-pentanol, 1-(2-hydroxymethoxy)-2-hexanol,1-(2-hydroxypropoxy)-2-octanol, 3-allyloxy-1,5-pentanediol,2-allyloxymethyl-2-methyl-1,3-pentanediol,(4,4-pentyloxy-methyl)-1,3-propanediol,3-(o-propenylphenoxy)-1,2-propanediol, 2,2'-diisopropylidenebis(p-phenyleneoxy) diethanol, glycerin, 1,2,6-hexanetriol,1,1,1-trimethyloethane,1,1,1-trimethylolpropane,3-(2-hydroxyethoxy)-1,2-propanediol,3-(2-hydroxypropyl)-1,2-propanediol,2,4-dimethyl-2-(2-hydroxyethoxy)-methylpentanediol-1,1,1,1-tris((2-hydroxyethoxy)methyl)-ethane, 1,1,1-tris ((2-hydroxypropoxy) methyl)propane, pentaerythritol, sorbitol, sucrose, lactose, α-methylglucoside, α-hydroxyalkyl glucosides, novolak resin, phosphoric acid,benzenephosphoric acid, polyphosphoric acid (such as tripolyphosphoricacid and tetrapolyphosphoric acid), and caprolactone may also beemployed.

Polyester polyols formed from one type, or two types or more, amongcompounds which possess at least two hydroxy groups, such aspolytetramethylene ether glycol, also ethylene glycol, diethyleneglycol, triethylene glycol, 1,2-propylene glycol, dipropylene glycol,trimethylene glycol, 1,3- and 1,4- butanediol, tetramethylene glycol,neopentyl glycol, hexamethylene glycol, decamethylene glycol, glycerin,trimethylolpropane, hexanetriol, pentaerythritol, and sorbitol, and onetype, or two types or more, among compounds which possess at least twocarboxyl groups, such as malonic acid, maleic acid, succinic acid,adipic acid, tartaric acid, pimelic acid, sebacic acid, oxalic acid,phthalic acid, terephthalic acid, hexahydrophthalic acid, aconitic acid,trimellitic acid, and hemimellitic acid can also be used.

Also ring-opened polymers of cyclic esters as polycaprolactone; andfurthermore, there are so-called polymer polyol compositions amongpolyether polyols and/or polyester polyols such as the commerciallyavailable polyols sold under the tradename NIAX® from UNION CARBIDECorporation, and also polyols which can be obtained by polymerizingethylenically unsaturated compounds, such as acrylonitrile, styrene,α-methylstyrene and methylmethacrylate in a polyether or polyesterpolyol. One can further use, for example, 1,2-polybutadiene glycol,1,4-polybutadiene glycol, polyhydroxy polyacrylate, and epoxy resins.

The polymeric isocyanate-terminated urethane precursors may be usedalone with a secondary hydroxyl-containing curing agent to form theadhesive compositions of this invention or they may be further reactedwith still another monomeric or polymeric compound containing hydroxylgroups as component (a)(ii). The useful prepolymer adducts should not begelled and should be soluble in the solvents used.

Component (a)(ii) may comprise any of the polyhydroxy compoundsdescribed above for forming the polymeric urethane precursors for use ascomponent (a)(i). In this way, for example, a polyether adduct may befurther reacted with a polyester polyol or polycaprolactone toincorporate desired end properties to the polyurethane adhesive, such asfor example, increased flexibility or elastomeric properties.

Curing agent, component (b), may comprise any monomeric or polymericcompound possessing at least three hydroxyl groups, at least one ofwhich comprises a secondary hydroxyl group and preferably not more thantwo of said hydroxyl groups are primary hydroxy. Illustrative curingagents for use in the adhesive compositions of the present inventioninclude glycerol, 1,2,6-hexanetriol, or polymeric polyols containingsecondary hydroxy groups or polyesters derived from secondary hydroxypolyols. Illustrative polymeric polyols which are presently commerciallyavailable, are sold under the tradenames NIAX® LHT 240 and NIAX® LS 490,both available from Union Carbide Corporation.

Components (a)(i), (a)(ii) and (b) may optionally be mixed or reacted ina suitable solvent.

As used herein, the term "suitable solvent" is defined as a solventwhich is relatively inert to the reactants. Suitable solvents includeN,N-dimethylformamide, N,N-dimethylacetamide, N,N-dimethylpropionamide,tetrahydrofuran, dimethyl sulfoxide, toluene, and mixtures thereof. Apreferred solvent is N,N-dimethylformamide. When a solvent is notrequired, it is generally preferred not to use one.

Optionally, the adhesive compositions of the subject invention mayadditionally contain an inert filler added as a thickener or thixotropeor to obtain a desired paste-like consistency. Suitable inert fillerswhich may be added for this purpose are talc, clay, titanium dioxide,lithopone, zinc oxide, zirconium silicate, silica aerogel, iron dioxide,diatomaceous earth, calcium carbonate, fumed silica, silazane treatedsilica, precipitated silica, glass fibers, magnesium oxide, chromicoxide, zirconium oxide, aluminum oxide, crushed quartz, calcined clay,asbestos, carbon, graphite, cork, cotton, synthetic fibers, etc.

Fumed colloidal silica is a preferred filler for use herein. Generally,a filler may be added in an amount ranging from about 10 to about 700parts by weight, based upon the combined weight of (a) and (b).Typically however, the filler or fillers are added in amounts of fromabout 10 to about 60 parts by weight, based upon the combined weight of(a) and (b).

The new and improved storage stable, one-package, heat curablepolyurethane adhesive compositions are simply prepared by mixing each ofthe components at room temperature to form a heat curable pastecomposition. In a preferred method, after the components have beenthoroughly admixed, the composition is degassed by heating thecomposition at a temperature of about 70° C. to about 100° C. undervacuum. The degassed composition may thereafter be stored in a closedcontainer in the absence of atmospheric moisture at room temperature fora period up to about six months.

The new and improved adhesive compositions of the subject invention maybe applied to a plurality of workpieces to be adhesively bonded by anysuitable means depending on the consistency of the composition, such asfor example by brushing, spraying or trowelling. Thereafter, when theworkpieces are brought into close proximity so as to form a sandwichstructure adhesive joint therebetween, and are heated, the compositioncures to form a solid firmly adhered adhesive joint exhibiting excellenttensile strength and bonding strength. Generally, the compositions ofthe present invention will be satisfactorily cured by heating theadhesive composition and the substrate(s) to a temperature of about 100°to about 200° C. for a period of from about 1 to 60 minutes. Preferably,the adhesive compositions of the subject invention and adhesive jointscomprising same are cured at from about 125°-155° C. for about 3 toabout 30 minutes. Actual curing times may vary depending temperature,shape of the materials, used and the heat transfer media employed. Priorto heating, the compositions of this invention exhibit excellent worklives and generally will not gel at room temperature for as long asabout six weeks.

In order that those skilled in the art may better understand how thepresent invention may be practiced, the following Examples are providedby way of illustration and not by way of limitation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

A paste was prepared by mixing, at room temperature, 2.34 grams (0.0033mole) of trifunctional polypropylene glycol (NIAX®LHT240), 4.6 grams(0.04 equivalent) of another trifunctional polyether (NIAX® LS490), 1.0gram of a thickening agent (CABOSIL®N 70 TS; Cabot Corporation), and 6.5grams (0.025 mole) of methylenebis(4-cyclohexylisocyanate) (DESMODUR®W;Mobay). In this mixture NIAX®LHT240 and LS490 provide 20% and 80%,respectively, of the stoichiometric amount of hydroxyl needed to reactwith the isocyanate groups. This mixture was storage-stable at roomtemperature for about four months.

The paste was applied to two cold-rolled steel laps (1"×5"), with anoverlap of one inch, using a fine wire to provide a 0.005" thickness tothe glueline. The laps were clamped and cured in an oven at 150° C. for60 minutes. The tensile strength of the laps after curing was 1620 psi.

EXAMPLE 2

A viscous liquid was prepared by mixing and warming 10.0 grams (0.087equivalent) of a polyether triol (NIAX®LS490), and 10.0 grams (0.082equivalent) of α, α, α', α'-tetramethyl-1,3-xylylene diisocyanate. Themixture was subsequently heated at 100° C. for 4 hours, and cooled toambient temperature. The resulting viscous liquid was storage-stable atroom temperature for six weeks.

The viscous liquid was applied to two cold-rolled steel laps asdescribed in Example 1, except that the cure time at 150° C. was variedfrom 10 minutes to 40 minutes. The tensile strengths obtained are shownbelow:

    ______________________________________                                                       Lap Shear                                                      Cure Time (mins.)                                                                            Tensil Strength (psi)                                          ______________________________________                                        10              40                                                            20              360                                                           30             1020                                                           40             1500                                                           ______________________________________                                    

EXAMPLE 3

An isocyanate-terminated prepolymer was prepared by heating 120 grams(1.2 equivalents) of a trifunctional polycaprolactone (NIAX®PCP301), 312grams (1.2 equivalents) of a difunctional polycaprolactone(NIAX®PCP200), and 600 grams (4.92 equivalents) of α, α,α'α'-tetramethyl-1,3-xylylene diisocyanate, at 90°-100° C. for 42 hours.The resulting prepolymer had an isocyanate content of 10.2%.

A paste was prepared by mixing 11.97 grams of the above prepolymer, 0.5gram of a thickening agent (CABOSIL®MS), and 0.89 gram of glycerol toprovide one equivalent of hydroxyl per equivalent of isocyanate group.The mixture was subsequently degassed under high vacuum to removetrapped gases.

The product was applied to two cold-rolled steel laps as described inExample 1, except that it was cured at 150° C. for 80 minutes. Thetensile strength obtained was 2650 psi.

Although the present invention has been described with reference tocertain preferred embodiments, it is apparent that modifications orchanges may be made therein by those skilled in this art withoutdeparting from the scope and spirit of the present invention as definedby the appended claims.

What is claimed is:
 1. A storage stable, one package heat curablepolyurethane adhesive composition, said composition comprising:(a) amixture of or a prepolymer adduct of:(i) a poly aliphatic isocyanatecompound of the formula: ##STR5## wherein R and R' are each,independently, selected from hydrogen, alkyl or substituted alkyl and Xis a bridging means selected from aliphatic, cycloaliphatic, or aromaticgroups, or a prepolymer adduct of said isocyanate compound possessingpoly aliphatic isocyanate functionality; and (ii) a monomeric orpolymeric compound containing a plurality of hydroxyl groups; whereincomponent (ii) is added in an amount so as to provide less than astoichiometrically equivalent amount of hydroxyl groups to saidaliphatic isocyanate groups present in (a), in admixture with (b) anamount of a curing agent comprising a monomeric or polymeric compoundcontaining at least three hydroxyl groups, at least one of said hydroxylgroups being a secondary hydroxyl group, said curing agent being presentin an amount sufficient to provide an approximately stoichiometricallyequivalent amount of hydroxyl groups to said excess of aliphaticisocyanate groups in (a); and (c) optionally, an effective amount of aninert filler.
 2. A polyurethane adhesive composition as recited in claim1, wherein component (a)(ii) is present in an amount such that component(a) possesses an overall stoichiometric excess of aliphatic isocyanategroups over hydroxyl groups of from about 5% to about 15% by weight ofthe total aliphatic isocyanate content of (a) (i).
 3. A polyurethaneadhesive composition as defined in claim 1 wherein in curing agentcomponent (b) not more than two of said hydroxyl groups are primaryhydroxy.
 4. A polyurethane adhesive composition as defined in claim 1wherein component (a)(i) comprises α, α, α', α'-tetramethylxylylenediisocyanate.
 5. A polyurethane adhesive composition as defined in claim1, wherein component (a)(i) comprises a polymeric adduct of a polyaliphatic isocyanate compound and a hydroxyl-terminated polyether, saidadduct having aliphatic isocyanate terminal groups.
 6. A polyurethaneadhesive composition as defined in claim 1, wherein component (a)(i)comprises a polymeric adduct of a poly aliphatic isocyanate compound anda hydroxyl-terminated polyester, said adduct having aliphatic isocyanateterminal groups.
 7. A polyurethane adhesive composition as defined inclaim 1, wherein said curing agent (b) is selected from the groupconsisting of glycerol, 1,2,6-hexanetriol, and polymeric polyolcompounds containing secondary hydroxyl groups.
 8. A polyurethaneadhesive composition as defined in claim 1, wherein inert filler (c) isa fumed colloidal silica filler which is present in an amount of about10 to about 60 parts by weight, based on the combined weight of (a) and(b).
 9. A method for preparing a storage stable, one package, heatcurable polyurethane coating composition, said method comprising:(a)forming a liquid mixture of a poly aliphatic isocyanate compound of theformula ##STR6## wherein R and R' are each, independently, selected fromhydrogen, alkyl and substituted alkyl and X is a bridging means selectedfrom divalent aliphatic, cycloaliphatic or aromatic groups, or apolymeric adduct of said aliphatic isocyanate compound possessing polyaliphatic isocyanate functionality, optionally in a suitable solvent;(b) adding to (a), a liquid mixture of a monomeric or polymeric compoundcontaining a plurality of hydroxyl groups, optionally in a suitablesolvent, in an amount so as to provide less than a stoichiometricallyequivalent amount of hydroxyl groups to the isocyanate groups present in(a) to provide a mixture or prepolymer containing from about 5% to about15% by weight of a stoichiometric excess of aliphatic isocyanateequivalents over hydroxyl equivalents in said mixture or prepolymer; (c)thereafter, adding a curing agent comprising a monomeric or polymericcompound containing at least three hydroxyl groups, at least one ofwhich is a secondary hydroxyl group, in an amount sufficient to providean approximately stoichiometrically equivalent amount of hydroxyl groupsto said excess of aliphatic isocyanate groups in the liquid mixture ofstep (b); and (d) optionally, adding an effective amount of an inertfiller to obtain a storage stable one package heat curable polyurethaneadhesive composition.
 10. A method as recited in claim 9, furtherincluding as step (e), the step of degassing the mixture of step (c) or(d) by heating said mixture at a temperature of about 70° to about 100°C. under a vacuum for a time sufficient to degas the composition.
 11. Anarticle of manufacture comprising a plurality of pieces each piecehaving at least a part of a surface portion in close proximity toanother piece, and having an adhesive joint therebetween, said adhesivejoint comprising a heat cured composition as defined in claim
 1. 12. Amethod of bonding a plurality of articles comprising:(a) providing aplurality of articles; (b) applying a layer of heat curable polyurethaneadhesive composition as defined in claim 1 to at least a portion of asurface of at least one of the articles; (c) bringing at least twosurfaces of at least two of said articles into close proximity so as tocomplete formation of a heat-curable adhesive joint therebetween; and(d) exposing the article of step (c) to elevated temperatures of about100° to about 200° C. for a time sufficient to cure said adhesivecomposition.